1. Field of the Invention
The present invention generally relates to materials comprising light-emitting semiconductor quantum dots (QDs). More particularly, it relates to silicone-based polymers incorporating QDs.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Light-emitting diodes (LEDs) are becoming more important in everyday life and it is envisaged that they will become one of the major applications in many forms of lighting such as automobile lights, traffic signals, general lighting, liquid crystal display (LCD) backlighting and display screens. Currently, LED devices are typically made from inorganic solid-state compound semiconductors, such as AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue). However, using a mixture of the available solid-state compound semiconductors, solid-state LEDs that emit white light cannot be produced. Moreover, it is difficult to produce “pure” colors by mixing solid-state LEDs of different frequencies. Therefore, the main, current method of color mixing to produce a required color, including white, is to use a combination of phosphorescent materials that are placed on top of the solid-state LED whereby the light from the LED (the “primary light”) is absorbed by the phosphorescent material and then re-emitted at a different frequency (the “secondary light”), i.e., the phosphorescent materials down convert the primary light to the secondary light. Moreover, the use of white LEDs produced by phosphor down-conversion leads to lower cost and simpler device fabrication than a combination of solid-state red-green-blue LEDs.
Current phosphorescent materials used in down converting applications absorb UV or mainly blue light and convert it to longer wavelengths, with most phosphors currently using trivalent rare-earth doped oxides or halophosphates. White emission can be obtained by blending phosphors that emit in the blue, green and red regions with that of a blue- or UV-emitting solid-state device. i.e., a blue light-emitting LED plus a green phosphor such as, SrGa2S4:Eu2+, and a red phosphor such as, SrSiEu2+ or a UV light-emitting LED plus a yellow phosphor such as, Sr2P2O7:Eu2+; Mn+2 and a blue-green phosphor. White LEDs can also be made by combining a blue LED with a yellow phosphor, however, color control and color rendering may be poor when using this methodology due to lack of tunability of the LEDs and the phosphor. Moreover, conventional LED phosphor technology uses down converting materials that have poor color rendering (i.e., color rendering index (CRI)<75).
There has been substantial interest in exploiting the properties of compound semiconductors consisting of particles with dimensions on the order of 2-50 nm, often referred to as quantum dots (QDs) or nanocrystals. These materials are of commercial interest due to their size-tuneable electronic properties that can be exploited in many commercial applications such as optical and electronic devices and other applications, including biological labeling, photovoltaics, catalysis, biological imaging, LEDs, general space lighting and electroluminescent displays, amongst many new and emerging applications.
The most studied of semiconductor materials have been the chalcogenides II-VI materials namely ZnS, ZnSe, CdS, CdSe, CdTe; most noticeably CdSe due to its tuneability over the visible region of the spectrum. Reproducible methods for the large scale production of these materials have been developed from “bottom up” techniques, whereby particles are prepared atom-by-atom, i.e., from molecules to clusters to particles, using “wet” chemical procedures.
Two fundamental factors, both related to the size of the individual semiconductor nanoparticle, are responsible for their unique properties. The first is the large surface-to-volume ratio; as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material. The second factor is, with many materials including semiconductor nanoparticles, there is a change in the electronic properties of the material with size, moreover, because of quantum confinement effects the band gap gradually becomes larger as the size of the particle decreases. This effect is a consequence of the so-called confinement of an “electron in a box” giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as observed in the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, because of the physical parameters, the “electron and hole”, produced by the absorption of electromagnetic radiation, a photon, with energy greater than the first excitonic transition, are closer together than they would be in the corresponding macro-crystalline material. Moreover, the Coulombic interaction cannot be neglected. This leads to a narrow bandwidth emission that depends upon the particle size and composition of the nanoparticle material. Thus, quantum dots have higher kinetic energy than the corresponding macro-crystalline material and consequently the first excitonic transition (band gap) increases in energy with decreasing particle diameter.
Core semiconductor nanoparticles that consist of a single semiconductor material along with an outer organic passivating layer tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface that may lead to non-radiative electron-hole recombinations. One method to eliminate defects and dangling bonds on the inorganic surface of the quantum dot is to grow a second inorganic material, having a wider band-gap and small lattice mismatch to that of the core material epitaxially on the surface of the core particle, to produce a “core-shell” particle. Core-shell particles separate any carriers confined in the core from surface states that would otherwise act as non-radiative recombination centers. One example is a ZnS shell grown on the surface of a CdSe core.
It will be appreciated from the foregoing discussion that many of the QD materials that have been extensively studied to date incorporate cadmium ions. There are, however, many environmental problems associated with the use of cadmium and other heavy metals such as mercury- and lead-based materials and thus there is a need to develop cadmium-free nanoparticle materials. In particular, it is desirable to produce non-cadmium containing quantum dots that exhibit similar monodispersities and size-tuneable photoluminescent spectra to current cadmium based materials. Commercial needs also dictate that such materials should be produced in high yields on a large-scale, as inexpensively as possible.
Rudimentary, quantum dot-based, light-emitting devices have been made by embedding colloidally produced quantum dots in an optically clear LED encapsulation medium, typically a silicone or an acrylate, which is then placed on top of a solid-state LED. The use of quantum dots potentially has certain significant advantages over the use of the more conventional phosphors, such as the ability to tune the emission wavelength, strong absorption properties, and low scattering (if the quantum dots are mono-dispersed).
For the commercial application of quantum dots in next-generation light-emitting devices, the quantum dots are preferably incorporated into the LED encapsulating material while remaining as fully mono-dispersed as possible and without significant loss of quantum efficiency. The methods developed to date are problematic, not least because of the nature of current LED encapsulants. Quantum dots can agglomerate when formulated into current LED encapsulants thereby reducing the optical performance of the quantum dots. Moreover, once the quantum dots are incorporated into the LED encapsulant, oxygen can migrate through the encapsulant to the surfaces of the quantum dots, which can lead to photo-oxidation and, as a result, a drop in quantum yield (QY).
One way of addressing the problem of oxygen migration to the QDs has been to incorporate the QDs into a medium with low oxygen permeability, such as a polymer. Polymers incorporating QDs can be used to make films or to make beads, which can be incorporated into light-emitting devices. However, QDs are not compatible with all polymer systems. In particular, cadmium-free QDs are difficult to match with compatible polymer systems. For example, incompatible polymer systems may react with QDs, causing the QY of the QD to decrease. Also, QDs tend to agglomerate in many polymer systems, causing the QY to decrease. To date, polymers based on acrylate monomers, such as methacrylates, have been found to be the most compatible with QDs. However, most acrylate systems are somewhat permeable to oxygen, and for that reason, are less than ideal. Also, acrylate polymers are subject to degradation by high temperatures, ultraviolet radiation, and oxidation.